1. Field of the Invention
The present invention relates to relay circuits and, in particular, to trip circuits for tripping a breaker in response to an overcurrent control signal.
2. Description of the Related Art
Relay circuits are widely used in many applications, such as power systems. Typical uses include protection of utility and industrial feeders and short circuit and overload protection for transformers and motors. Such relays typically include both overcurrent detection circuitry that generates a trip control signal after overcurrent is detected and a trip circuit to energize a breaker trip circuit when the trip control signal is generated. The overcurrent detected may be based on a time or instantaneous overcurrent. Thus, for example, whenever an overcurrent is detected, the overcurrent detection circuitry generates a trip control signal, which is applied to the trip circuit. When the trip circuit receives the trip control signal, the trip circuit causes an appropriate circuit breaker to trip, thus protecting the device or system in which an overcurrent has been detected.
In prior trip circuits, the trip circuit may include a set of relay contacts and a relay coil coupled to the trip control signal. When the trip control signal is applied to the relay coil, the relay contacts are closed, thereby providing a current path for a large current to flow through the closed relay contacts and through a trip coil of the circuit breaker, thereby causing the circuit breaker to trip after the current flows through the circuit breaker trip coil for a sufficient duration of time.
One disadvantage of such trip circuits is that the relay contacts must be large enough to handle the large magnitude of current that is required to flow therethrough in order to trip the circuit breaker. Therefore, the trip control signal must supply a relatively large amount of power to the relay coil in order to close the relay contacts. This power requirement can be undesirable, since the relay system containing the overcurrent detection circuitry and trip circuit often must be self-powered. Additionally, the relay contacts must be held in the closed position by the relay coil until the circuit breaker trips. This increases the power required to trip the breaker, since the trip control signal must be applied to the relay coil until the circuit breaker trips. Further, if the trip control signal is removed from the relay coil for any reason before the circuit breaker trips, the relay contacts may be damaged because they are not normally rated to interrupt the circuit breaker trip coil circuit.
There is a need for improved trip circuits that have lower power requirements and that do not require trip control signals of as long a duration. There is also a need for methods and apparatuses for testing whether such circuits are functioning properly.